In an integrated circuit, analog, digital, or mixed-signal circuits have a range of power supply voltage within which they operate predictably and reliably. Consequently, integrated circuits typically include a power supply voltage detection circuit for monitoring the power supply during its power up sequence. More specifically, during the power up sequence of an integrated circuit, the power supply voltage may take hundreds of milliseconds to reach its desired value. Additionally, the power supply voltage may ramp up in a non-monotonic fashion, i.e., the power supply voltage may wobble up and down slightly as it is ramping up, until it reaches a stable desired voltage level. This is because during the power up sequence various circuits activate in sequence and may cause the demands on the power supply to vary in a nonlinear fashion. Consequently, the voltage detection circuit is utilized to detect when the power supply voltage has reached a certain minimum value and to generate an electronic indicator (e.g., a trigger flag) to the one or more circuits of interest, which is an indicator that a safe minimum operating voltage is reached.
Traditional power supply voltage detection circuits often trigger off of multiples of the voltage threshold (Vt) of a device, such as the Vt of a field-effect transistor (FET) device. In this scenario, when the power supply voltage reaches a value of, for example, Vt×1 or Vt×2, a trigger flag is generated. However, the Vt of devices changes with process, voltage, and temperature variations and, thus, using a multiple of Vt is not a stable way to establish a voltage detection circuit. More specifically, because the Vt value may vary ±300-450 mV with process, voltage, and temperature, the trigger voltage that results from of a stack of transistors, which is used to generate multiples of Vt, may vary over several hundred millivolts (mV).
For at least these reasons, a need exists for a voltage detection circuit in an integrated circuit and method of generating a trigger flag signal, in order to provide a voltage detection circuit that has a more predictable and stable trigger flag signal as compared with traditional Vt-based voltage detection circuits.